1. Field of the Invention
This invention relates generally to radio receivers. More particularly, the invention relates to low intermediate frequency receivers.
2. Related Art
Simple textbook models of radio receivers typically use homodyne detection. Homodyne detection involves directly demodulating a radio frequency (RF) signal to baseband in a single operation. A receiver that implements homodyne detection is commonly referred to as a direct conversion receiver (DCR). Current state of the art radio receivers for Global System for Mobile Communication/Enhanced Data Rates for GSM Evolution (GSM/EDGE) use DCR systems at least in the EDGE mode. DCR systems have traditionally not been employed in practical applications until recently. This is largely due to problems with direct current (DC) offset. DC offset generally refers to the DC voltage at the output of a system that is unrelated to the signal at the input to the system. An interfering signal (or signals) at the carrier frequency may, after demodulation, cause a DC offset to be added to the desired baseband signal. The interfering signal may include a self-generated interferer component caused by LO self mixing (self-interference from the local oscillator or LO), among other causes. In a DCR system, this interferer component is often manifested as a complex DC offset. In EDGE modulation, for example, DC offset that is added to the signal can degrade performance of portable transceivers.
More particularly, DC offset can impair received signals, especially at low-signal levels, often resulting in fundamental limitations in EDGE systems in particular and digital communications in general. Solutions to reduce added DC offset often introduce non-linearity at low-signal levels, add increased componentry (and thus consume more space and power), and may add to the complexity (and cost) of a system. For example, DC offset is often removed in DCR systems by implementing an averaging and subtraction process on a burst by burst basis. A burst is a defined time interval that varies depending on the mode used and/or as configured by a user. For example, in GSM systems, signals arrive in bursts of approximately 577 microseconds in duration. Although this averaging/subtraction process may work for GSM, EDGE presents different challenges. In EDGE, the required signal to noise ratio (SNR) to support the channel bit rate is much higher than in GSM, which typically requires more complex techniques to remove DC offset. In a time-division multiple access (TDMA) system such as GSM/EDGE, the phase of an added DC offset may change somewhat randomly from burst to burst, making long term averaging of DC offsets difficult.
A further problem that can arise with DCR systems is in the manufacturing process. In solid state devices a phenomenon known as 1/f noise adds a low frequency noise to the desired signal. Semiconductor manufacturers attempt to address this problem in DCR chip fabrication by using a process with good 1/f noise characteristics. Semiconductor fabrication processes with good 1/f noise characteristics are relatively expensive.
To avoid the above described problems associated with a DCR system, it is possible to use a superheterodyne receiver. In a superheterodyne receiver, a RF signal is converted to an intermediate frequency (IF) where DC offset can often be easily removed. Then the IF signal is converted to baseband to provide the desired signal at baseband with substantially no DC offset. Conventional IF-based radios use IF frequencies that are many multiples of the desired signal bandwidth to ensure that rejection of image frequencies is adequately high. This approach adds a significant cost to a radio as it normally requires expensive filtering components as well as a second set of mixer hardware.
Some radio architectures use a low IF receiver. In this case, the intermediate frequency is chosen to be of the same or of similar value to the bandwidth of the desired signal. This approach allows the received signal to be demodulated in one operation (as in DCR systems) to a low intermediate frequency which can be digitized, and thus the final low IF conversion (which converts the desired signal to baseband) can be carried out in the digital domain. For example, in a GSM system, it is possible to demodulate to a low intermediate frequency before digitizing the signal, and then do the final demodulation in the digital domain. A low IF architecture achieves a similar benefit as DCR systems, including minimal RF hardware while minimizing the effects of DC offset and 1/f noise because such effects are no longer at the center of the desired signal bandwidth. In practice, this can sometimes result in cheaper semiconductor processes for low IF systems versus DCR systems.
Some problems common to DCR systems may still exist in low IF receiver systems. For example, a large self-generated interferer component (e.g., caused by LO self mixing) may still be present, manifested as an interferer at the low intermediate frequency. FIG. 1 provides an illustration of an exemplary frequency environment in which interferers can be manifested, and which will assist in understanding the nature of some of the problems that may occur in low IF receiver systems. Shown is a frequency domain plot 100 that may be illustrative of channel slots for a mobile telephone communicating with a base station in a GSM system. The frequency domain plot 100 has an x-axis 102 in units of kilo-Hertz (kHz). For example, frequencies to the right-hand side of the desired signal take on negative frequency values, and frequencies to the left-hand side of the desired signal take on positive frequency values. The sign (positive or negative) is a function of the 90-degree separation in the signal that occurs at a quadrature mixer in a low IF receiver system. As is well known, converting a signal down to baseband involves a multiplication by a complex rotation (e.g., cosine and sine multiplication). If the two signals (cosine and sine) are in perfect quadrature (i.e., exactly 90-degree separation), a demodulation operation results in a positive adjacent and negative adjacent signal in distinct channel slots 104 (i.e. no folding over of positive frequencies onto negative frequencies and vice versa). If the two signals (cosine and sine) are not in perfect quadrature (i.e., non-90-degree separation), then fold over of complementary (opposite in sign) frequencies begins to occur. The greater the deviation from ideal (e.g., 90-degrees), the greater the leakage or fold over from other channels. Along the x-axis 102 are a plurality of channel slots 104 for channels that each have a bandwidth of 200 kHz. For example, shown on each side of the desired signal centered at 100 kHz are 1st adjacent channels, 2nd adjacent channels, and so forth. In low IF receivers operating in an interference limited environment such as GSM, receiver impairments (e.g., gain imbalance, phase imbalance, etc.) may cause some portion of the energy of adjacent channel interferers (in particular, the energy of the opposite sideband of the 2nd adjacent channel since fold over due to RF impairments is symmetric about 0 Hertz (Hz)) to fall directly in the bandwidth of the received signal and thereby degrade receiver performance. A further problem is that typical low IF systems for GSM use a low intermediate frequency of 100 kHz, which may result in a 100 kHz interferer tone added to the desired signal at the radio output. A notch filter can be used to attenuate the energy of this tone without significantly impacting system performance. With EDGE modulation, however, the use of a notch filter to remove the 100 kHz tone may degrade system performance.
Thus, it would be desirable to have a receiver architecture or system that efficiently mitigates or removes interferers and/or DC offset in a low IF system while minimizing cost, size, and power consumption of the receiver.